In order to avoid the damage that a wringer can do to laundry, and the difficulties of properly distributing the load in a centrifugally acting extractor, it is known to express the water from a load of laundry by means of a piston-type machine. Such an apparatus in a single operation compresses the load of laundry, physically forcing the liquid from it.
In one such apparatus the laundry is loaded into an upwardly open vessel whose lower wall is perforated and connected to a drain. A piston can slide down inside the chamber and compress the load against the floor, thereby expressing the water from it. Such an apparatus is difficult to load and unload, and does not subject all the load to the same amount of compression. Thus bulkier parts of the load are compressed more, and more liquid is expressed from them, than less bulky parts.
A partial solution to this problem is described in U.S. Pat. No. 1,985,485. This apparatus has a spherical chamber which can be closed around the load of laundry. The floor of the chamber is perforated and provided with a drain and the upper chamber wall is flexible and can be forced piston-fashion down against the load of a laundry sitting on the bottom wall to express the liquid from it. Such an apparatus does indeed subject the entire load to a uniform pressure, but is fairly complex. The membrane can be damaged by sharp objects in the laundry, and it is necessary to provide complicated locking devices to hold the chamber closed when the compartment above the membrane is inflated. In addition this arrangement must be manually loaded and unloaded, so it is only usable in a labor-intensive laundry-processing operation.
In order to protect the membrane it is known to overlie the load of laundry with a thick blanket that is of some tough and flexible material that prevents sharp objects in the load being dewatered from piercing the membrane. Such an overlay transfers much of the downwardly effective force into outwardly effective force on the side walls of the laundry-holding vessel, requiring corresponding sturdy construction of these parts. A secondary piston can be used, in order to exert the 20 bar to 30 bar of pressure needed to express the liquid from the laundry but even so drying is often fairly uneven and the device is subject to considerable wear, so its service life is short.